Visible light communication (VLC) is a wireless communication technology which uses a visible light source to emit high frequency visible light signal for signal transmission. The frequency for the visible light is between 400 THz (having a wavelength of 780 nm) to 800 THz (having a wavelength of 375 nm). When a general fluorescent lamp is used as the light source, the transmission capacity is about 10 Kbit/s. When an LED (light emitting diode) is used as the light source, the transmission capacity is about 500 Mbit/s. Besides, the transmission distance may be up to 1 or 2 kilometers.
The VLC technology suitable for in indoor/outdoor positioning, hospital equipment or office communication safety etc. Besides, the VLC technology has high security issue because only when the receiver is close enough to the transmitter for receiving the visible light signal from the transmitter, the communication is allowed. The VLC technology has another issue in that the VLC is not interfered by the radio frequency signals.
However, as for the VLC technology, before communication, both the receiver and the transmitter do not know the central frequency and the usable bandwidth of the visible light source. That is because there are many types for the visible light sources which may have different central frequency and usable bandwidth.
FIG. 1A to FIG. 1D show measurements of the central frequency and usable bandwidth of four types of fluorescent lamp. The usable bandwidth in FIG. 1A to FIG. 1D is 10 MHz, 9 MHz, 2 MHZ and 5 MHz, respectively; and the central frequency in FIG. 1A to FIG. 1D is 7 MHz, 7.5 MHz, 9 MHZ and 9 MHz, respectively. Thus, even the common fluorescent lamp is used as the visible light source for VLC, the difference between the usable bandwidth of different common fluorescent lamps may be large. Further, as shown in FIG. 1A to FIG. 1D, the central frequency of the different fluorescent lamps is also different.
Thus, how to find the central frequency and the usable bandwidth of the visible light source in VLC quickly and accurately is an important issue.